[leetcode 99] Recover Binary Search Tree

Two elements of a binary search tree (BST) are swapped by mistake.

Recover the tree without changing its structure.
Note:

A solution using O(n)
space is pretty straight forward. Could you devise a constant space solution?

Morris中序遍历

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
void recoverTree(TreeNode *root) {
if (!root) {
return ;
}
pair<TreeNode*, TreeNode*> reorder;
TreeNode *prev = NULL;
TreeNode *cur = root;
while(cur) {
if (!cur->left) {
detect(reorder, prev, cur);
prev = cur;
cur = cur->right;
} else {
auto node = cur->left;
while (node->right && node->right != cur) {
node = node->right;
}
if (!node->right) {
node->right = cur;
cur = cur->left;
} else {
detect(reorder, prev, cur);
node->right = NULL;
prev = cur;
cur = cur->right;
}
}
}
swap(reorder.first->val, reorder.second->val);
return ;
}
void detect(pair<TreeNode*, TreeNode*> &reorder, TreeNode* prev, TreeNode *cur) {
if (prev && prev->val >= cur->val) {
if (!reorder.first) {
reorder.first = prev;
}
reorder.second = cur;
}
}
};